Most Popular
1500 questions
38
votes
11 answers
Recommended books for advanced undergraduate electrodynamics
What books are recommended for an advanced undergraduate course in electrodynamics?
A.khalaf
- 852
38
votes
19 answers
Why does the gas get cold when I spray it?
When you spray gas from a compressed spray, the gas gets very cold, even though, the compressed spray is in the room temperature.
I think, when it goes from high pressure to lower one, it gets cold, right? but what is the reason behind that…
Ken D
- 491
38
votes
4 answers
Why is Earth's climate so stable?
Earth wasn't always the only water-world in the solar system. Mars also appear to have started out wet
but, as conditions changed, Mars lost its oceans.
So, how has Earth managed to avoid a similar fate?
Doesn't the Giant impact hypothesis…
Achmed
- 1,139
38
votes
5 answers
What are washers for?
When you attach a bolt to something using a nut, it is clear what the roles of the nut and bold are.
The more you tighten the bolt the more secure your fastening. However, you are often also told to use a washer as well.
I know this somehow…
Simd
- 1,151
38
votes
8 answers
Interval preserving transformations are linear in special relativity
In almost all proofs I've seen of the Lorentz transformations one starts on the assumption that the required transformations are linear. I'm wondering if there is a way to prove the linearity:
Prove that any spacetime transformation…
a06e
- 3,840
- 4
- 41
- 75
38
votes
5 answers
Why isn't it $E \approx 27.642 \times mc^2$?
Why is it that many of the most important physical equations don't have ugly numbers (i.e., "arbitrary" irrational factors) to line up both sides?
Why can so many equations be expressed so neatly with small natural numbers while recycling a…
badroit
- 499
38
votes
4 answers
Birds sitting on electric wires: potential difference between the legs
We have seen birds sitting on uninsulated electric wires of high voltage transmission lines overhead without getting harmed, because sitting on only one wire doesn't complete any circuit.
But what about the potential difference between their legs?…
Tea is life
- 2,804
38
votes
5 answers
Equivalence between Hamiltonian and Lagrangian Mechanics
I'm reading a proof about Lagrangian => Hamiltonian and one part of it just doesn't make sense to me.
The Lagrangian is written $L(q, \dot q, t)$, and is convex in $\dot q$, and then the Hamiltonian is defined via the Legendre transform
$$H(p,q,t) =…
Mark
- 559
- 5
- 13
37
votes
3 answers
Eigenfunctions of the Runge-Lenz vector
The hamiltonian for the hydrogen atom,
$$
H = \frac{\mathbf{p}^2}{2m} - \frac{k}{r}
$$
is spherically symmetric and it therefore commutes with the angular momentum $\mathbf{L}$; this causes all its eigenfunctions with equal angular momentum number…
Emilio Pisanty
- 137,480
37
votes
6 answers
Fundamental invariants of the electromagnetic field
It is a standard exercise in relativistic electrodynamics to show that the electromagnetic field tensor $F_{\mu\nu}$, whose components equal the electric $E^i=cF^{i0}$ and magnetic $B_i=-\frac12\epsilon_{ijk}F^{jk}$ fields in the taken frame of…
Emilio Pisanty
- 137,480
37
votes
5 answers
What is so special about speed of light in vacuum?
I will try to be as explanatory as possible with my question. Please also note that I have done my share of googling and I am looking for simple language preferable with some example so that I can get some insight in this subject.
My question is…
java_doctor_101
- 673
37
votes
9 answers
Why do disk brakes generate "more stopping power" than rim brakes?
Bicycle's brakes generally fall into two broad categories: rim brakes where the braking pads pinch against the wheel's rim, and disk brakes where the pads press against a disk (rotor) that is rigidly attached to the wheel's hub.
General wisdom is…
Kevin Kostlan
- 6,875
- 1
- 25
- 36
37
votes
6 answers
What is canonical momentum?
What does the canonical momentum $\textbf{p}=m\textbf{v}+e\textbf{A}$ mean? Is it just momentum accounting for electromagnetic effects?
cpc333
- 1,157
- 3
- 12
- 19
37
votes
4 answers
Floating a ping pong ball in the air just using a pen!
Caution: Apparently this problem is harder than it seems!
There is a well known phenomena, which I first learnt about when I was a 10 years old kid. You can levitate a ping pong(or whatever not-so-heavy) ball in the air by blowing throw an empty…
Ali
- 6,052
- 6
- 36
- 50
37
votes
4 answers
Why do we need insulation material between two walls?
Consider a slab made of two walls separated by air. Why do we need insulation material between the two walls. Air thermal conductivity is lower than most thermal conductivities of insulating material and convection cannot be an issue in the enclosed…
Shaktyai
- 2,020